In the production of printed circuit boards, a dry film photoresist as disclosed in Japanese Patent Publication No. 25231/70 can be laminated on a copper-clad laminate (base) and exposed to actinic light through a mask having, e.g., a wiring pattern, and the unexposed areas are dissolved using an adquate developing solution, thereby obtaining a cured image on the base. The resulting cured image is used as a resist, and the exposed copper which is not covered by the resist is then etched, followed by plating, anodization, etc., to thereby obtain a printed circuit board.
However, the dry film resist has a poor adhesiveness to a metal surface as compared with a solution type photoresist, resulting in various unfavorable phenomenon in the etching or plating. For example, when the base is sprayed with an etching solution or dipped in a plating solution, the solution infiltrates into the interface between the resist and the base, causing the resist to separate from the base. This leads to phenomenon, such as underetching or underplating, which, in turn, causes the edges of the image to become blurred or the resist image to disappear. As a result, the desired pattern is not obtained and many bases are thus wasted.
In the production of printed circuit boards by the use of a dry film resist, the resist pattern is closely attached to the base over the entire surface thereof in some cases. In other cases, the base has through-holes, with both sides of the base and the inside surfaces of the through-holes being coated with a metal layer such as a copper layer, and printed circuits are formed on both sides of the base which are electrically connecting with each other via the inside of the through-holes. In such cases, it is necessary that the resist pattern is not attached to the base over the entire surface thereof, and a resist film is spread on or beneath the holes over the openings of the holes (i.e., tenting) so as to protect the metal layer on the inside of the holes from etching. In such tenting, the resist film thus formed is attached to the base only at a small area around the openings of the base, with the film covering the openings of the holes being supported by its own cohesive force. Therefore, the resist film is required to have sufficient peel-resistance against the etching spray.
In order to improve the peel-resistance, pretreatment of the metal surface was proposed (Japanese Patent Publication No. 5292/79 corresponding to U.S. Pat. No. 3,873,316, and Japanese Patent Application (OPI) Nos. 64919/76 and 64920/76 ("OPI" as used herein means published unexamined application) corresponding to U.S. Pat. No. 4,233,395.
Several other methods for improving the adhesion by adding a variety of compounds to the photosensitive resin layer have also been proposed, as described, for example, in Japanese Patent Publication Nos. 9177/75, 5292/79 and 22481/80, Japanese Patent Application (OPI) Nos. 64919/76, 64920/76, 63087/75, 2724/77, 702/78, 124541/78, 124594/78, 133585/79, 133586/79 and 65947/80, Japanese Patent Publication Nos. 46053/82 and 46054/82, Japanese Patent Application (OPI) No. 11904/81, Japanese Patent Publication No. 21697/82, Japanese Patent Application (OPI) Nos. 75642/81 and 67844/81, Japanese Patent Publication No. 40500/82, Japanese Patent Application (OPI) Nos. 99202/81, 100803/81, 60327/82 and 62047/82.
Since the surface treatment in the former method requires an additional step, it would appear that the latter methods using additives are superior. However, the latter methods are disadvantageous in that the copper surface which is exposed after development turns red, and this discoloration adversely affects subsequent steps such as etching, plating, and soldering; furthermore, some additives discolor print-out dyes, interfere with the photopolymerization reaction, and/or have low miscibility with the photopolymerizable resin composition.